Framed on a wall in my office is a personal letter to me from Bill Gates the elder. "I am a fan of progressive taxation," he wrote. "I would say our country has prospered from using such a system—even at 70% rates to say nothing of 90%."
It's one thing to believe in bad policy. It's quite another to push it on others. But Mr. Gates Sr.—an accomplished lawyer, now retired—and his illustrious son are now trying to have their way with the people of the state of Washington.
Mr. Gates Sr. has personally contributed $500,000 to promote a statewide proposition on Washington's November ballot that would impose a brand new 5% tax on individuals earning over $200,000 per year and couples earning over $400,000 per year. An additional 4% surcharge would be levied on individuals and couples earning more than $500,000 and $1 million, respectively.
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Along with creating a new income tax on high-income earners, Initiative 1098 would also reduce property, business and occupation taxes. But raising the income tax is the real issue. Doing so would put the state's economy at risk.

To imagine what such a large soak-the-rich income tax would do to Washington, we need only examine how states with the highest income-tax rates perform relative to their zero-income tax counterparts. Comparing the nine states with the highest tax rates on earned income to the nine states with no income tax shows how high tax rates weaken economic performance.

In the past decade, the nine states with the highest personal income tax rates have seen gross state product increase by 59.8%, personal income grow by 51%, and population increase by 6.1%. The nine states with no personal income tax have seen gross state product increase by 86.3%, personal income grow by 64.1%, and population increase by 15.5%.
It's striking how the high-tax states have underperformed relative to those with no income tax. Especially noteworthy is how well Washington has performed compared to states with no income tax.

If Washington passes Initiative 1098, it will go from being one of the fastest-growing states in the country to one of the slowest-growing. And passage of I-1098 will only be the beginning. Just look at Ohio, Michigan and California to see that once a state adopts an income tax, there is no end to the number of reasons that such a tax could be extended, expanded and increased.

Over the past 50 years, 11 states have introduced state income taxes exactly as Messrs. Gates and their allies are proposing—and the consequences have been devastating.

Each and every state that introduced an income tax saw its share of total U.S. output decline. Some of the states, like Michigan, Pennsylvania and Ohio, have become fiscal basket cases. As the nearby chart shows, even West Virginia, which was poor to begin with, got relatively poorer after adopting a state income tax.

Washington's I-1098 proposes a state income tax with a maximum rate higher than any of those initially adopted by the other 11 states. In one fell swoop, Washington would move from being one of the lowest-tax states in the nation to being one of the top nine highest. It's economic suicide.

The states that have high income tax rates or have adopted a state income tax over the past 50 years haven't even gotten the money they hoped for. They haven't avoided budget crises, nor have they provided better lives for the poor. The ongoing financial travails of California, New Jersey, Ohio, Michigan and New York are cases in point.

Over the past decade, the nine states with the highest tax rates have experienced tax revenue growth of 74%—a full 22% less than the states with no income tax. Washington state has done better than the average of the nine no-tax states. Why on earth would it want to introduce a state income tax when it means less money for state coffers?

What's true for those states with the highest tax rates is doubly true for the 11 states that have instituted state income taxes over the past half-century. They too have lost huge sums of tax revenue.

A final thought for those who want to punish the rich for their success: As the nearby chart shows, those states with the highest tax rates, and those states that have introduced state income taxes, have seen standards of living (personal income per capita) substantially underperform compared to their no-tax counterparts.

If Mr. Gates Sr. and his son feel so strongly about taxing the rich, they should simply give the state a chunk of their own money and be done with it. Leave the rest of Washington's taxpayers alone.

Mr. Laffer is the chairman of Laffer Associates and co-author of "Return to Prosperity: How America Can Regain Its Economic Superpower Status" (Threshold, 2010).